Hello MMD,

 [ Carl Zwanzig wrote in 1100706 MMDigest:

> Generally speaking, there should be at least 1 megohm resistance
> from winding to case, ...

Totally correct!  In this respect one should look for factors that
caused the degradation of insulation resistance.

When the instrument has been stored in a damp environment for longer
terms, normal household dust can be responsible.  Another common cause
in motors with a collector may be carbon dust.

So if the ground fault is in the megohm region, cleaning the motor may
be a solution.  Professionals use a solvent in combination with an
ultrasonic bath.

Best regards from Germany,
Walter Tenten